1. Field of the Invention
This invention relates to a surface plasmon sensor for quantitatively analyzing a material in a sample utilizing generation of surface plasmon, and more particularly to a surface plasmon sensor in which the photodetecting portion is improved.
2. Description of the Related Art
In metal, free electrons vibrate in a group to generate compression waves called plasma waves. The compression waves generated in a metal surface are quantized into surface plasmon.
There have been proposed various surface plasmon sensors for quantitatively analyzing a material in a sample utilizing a phenomenon that such surface plasmon is excited by light waves. Among those, one employing a system called xe2x80x9cKretschmann configurationxe2x80x9d is best known. See, for instance, Japanese Unexamined Patent Publication No. 6(1994)-167443.
The plasmon sensor using the Kretschmann configuration basically comprises a dielectric block shaped, for instance, like a prism, a metal film which is formed on one face of the dielectric block and is brought into contact with a sample, a light source emitting a light beam, an optical system which causes the light beam to enter the dielectric block so that the light beam is reflected in total reflection at the interface of the dielectric block and the metal film and various angles of incidence of the light beam to the interface of the dielectric block and the metal film including an angle of incidence at which surface plasmon is generated can be obtained, and a photodetector means which is able to detect the intensity of the light beam reflected in total reflection from the interface for the various angles of incidence.
In order to obtain various angles of incidence of the light beam to the interface, a relatively thin incident light beam may be caused to impinge upon the interface while deflecting the incident light beam or a relatively thick incident light beam may be caused to converge on the interface so that components of the incident light beam impinge upon the interface at various angles. In the former case, the light beam which is reflected from the interface at an angle which varies as the incident light beam is deflected may be detected by a photodetector which is moved in synchronization with deflection of the incident light beam or by an area sensor extending in the direction in which reflected light beam is moved as a result of deflection. In the latter case, an area sensor which extends in directions so that all the components of light reflected from the interface at various angles can be detected by the area sensor may be used.
In such a plasmon sensor, when a light beam impinges upon the metal film at a particular angle of incidence xcex8sp not smaller than the angle of total internal reflection, evanescent waves having an electric field distribution are generated in the sample in contact with the metal film and surface plasmon is excited in the interface between the metal film and the sample. When the wave vector of the evanescent waves is equal to the wave number of the surface plasmon and wave number matching is established, the evanescent waves and the surface plasmon resonate and light energy is transferred to the surface plasmon, whereby the intensity of light reflected in total reflection from the interface of the dielectric block and the metal film sharply drops.
When the wave number of the surface plasmon can be known from the angle of incidence xcex8sp at which the phenomenon of attenuation in total reflection takes place, the dielectric constant of the sample can be obtained. That is,       Ksp    ⁢          xe2x80x83        ⁢          (      ϖ      )        =            ϖ      c        ⁢                                        ε            m                    ⁢                      xe2x80x83                    ⁢                      (            ϖ            )                    ⁢                      xe2x80x83                    ⁢                      ε            s                                                              ε              m                        ⁢                          xe2x80x83                        ⁢                          (              ϖ              )                                +                      ε            s                              
wherein Ksp represents the wave number of the surface plasmon, xcfx89 represents the angular frequency of the surface plasmon, c represents the speed of light in a vacuum, and xcex5m and xcex5s respectively represent the dielectric constants of the metal and the sample.
When the dielectric constant xcex5s of the sample is known, the concentration of a specific material in the sample can be determined on the basis of a predetermined calibration curve or the like. Accordingly, a specific component in the sample can be quantitatively analyzed by detecting the angle of incidence xcex8sp at which the intensity of light reflected in total reflection from the interface of the prism and the metal film sharply drops (This angle of incidence xcex8sp will be referred to as xe2x80x9cthe total reflection cancel anglexe2x80x9d, hereinbelow).
In the conventional plasmon sensor of the type described above, the intensity of the light beam reflected in total reflection from the interface for the various angles of reflection is detected by a photodetector which is moved in synchronization with deflection of the incident light beam, or an area sensor extending in the direction in which reflected light beam is moved as a result of deflection.
In the case of the former, though a relatively wide dynamic range can be ensured with respect to measurement of the total reflection cancel angle, it becomes difficult to effect analysis of the sample at a high speed since a mechanical drive mechanism for driving the photodetector becomes necessary.
In the case of the latter, though high speed analysis can be realized, it is difficult to ensure high accuracy in analysis since the area sensor such as a CCD is narrow in dynamic range for resolution and charge accumulation.
We have proposed an arrangement for detecting the total reflection cancel angle xcex8sp in which the amount of reflected light in a first reflecting angle range and the amount of reflected light in a second reflecting angle range are separately detected by a pair of photodetectors such as the segments of a two-segment photodiode and the total reflection cancel angle xcex8sp is obtained on the basis of a comparison of the outputs of the photodetectors. See Japanese Unexamined Patent Publication No. 9(1997)-292334.
With this arrangement, the total reflection cancel angle xcex8sp can be obtained at high sensitivity. However, in this case, measurement of the total reflection cancel angle xcex8sp becomes impossible when the total reflection cancel angle xcex8sp varies beyond the range where the photodetectors can receive the reflected light and as a result, the dynamic range is only 2 to 3xc2x0 at most.
In view of the foregoing observations and description, the primary object of the present invention is to provide a surface plasmon sensor in which the total reflection cancel angle xcex8sp can be measured at a sufficiently high accuracy with a wide dynamic range.
The surface plasmon sensor of the present invention comprises
a dielectric block shaped, for instance, like a prism,
a metal film which is formed on one face of the dielectric block and is brought into contact with a sample,
a light source emitting a light beam,
an optical system which causes the light beam to enter the dielectric block and converges the light beam on the interface of the dielectric block and the metal film so that components of the light beam impinge upon the interface at various angles including angles of total reflection,
a photodetector means which consists of an array of a plurality of photodetector elements extending in a predetermined direction and is positioned so that the components of the light beam reflected at the interface in total reflection at various angles are received by the respective photodetector elements,
a differential means which differentiates light detecting signals output from the photodetector elements with respect to the direction of the array of the photodetector elements, and
an operation means which determines the reflecting angle at which the intensity of the light reflected at the interface takes a minimum value on the basis of the differential values of the light detecting signals.
Preferably, the operation means is arranged to determine the reflecting angle at which the intensity of the light reflected at the interface takes a minimum value by interpolating a plurality of differential values in the range between the reflecting angle at which the differential value takes a maximum value and that at which the differential value takes a minimum value and determining the position in the direction of the array of the photodetector elements in which the differential value is zero.
As the differential means, one which obtains the difference between the light detecting signals output from a pair of adjacent photodetector elements can be suitably used. Further as the photodetector means, a photodiode array may be suitably used.
In the surface plasmon sensor with the arrangement described above, the incident angle xcex8 of the light beam to the interface of the dielectric block and the metal film corresponds to the position of the photodetector means in the direction of the array of the photodetector elements in one-to-one correspondence. Further, the intensity of the reflected light takes a minimum value at the total reflection cancel angle xcex8sp.
The position of the photodetector means in the direction of the array of the photodetector elements in which the intensity of the reflected light takes a minimum value can be determined on the basis of the differential values obtained by differentiating the light detecting signals output from the photodetector elements with respect to the direction of the array of the photodetector elements. That is, the differential value becomes zero in the position of the photodetector means in the direction of the array of the photodetector elements in which the intensity of the reflected light takes a minimum value.
Accordingly, the incident angle xcex8 of the light beam in one-to-one correspondence to the position of the photodetector means in the direction of the array of the photodetector elements in which the intensity of the reflected light takes a minimum value is the total reflection cancel angle xcex8sp, and a specific component in the sample can be quantitatively analyzed on the basis of the total reflection cancel angle xcex8sp.
Further since a photodiode array or the like is used as the photodetector means for detecting the intensity of the reflected light, the total reflection cancel angle xcex8sp can be measured at a high sensitivity.
Further since the array of the photodetector elements extends in the direction in which the total reflection cancel angle xcex8sp varies, the total reflection cancel angle xcex8sp can be detected even if it varies by a large amount so long as the variation is within the range where the array of the photodetector elements extends, which range can be theoretically extends as long as desired. Thus, in the surface plasmon sensor of the present invention, the total reflection cancel angle xcex8sp can be measured with a wide dynamic range.
Further, in the surface plasmon sensor of the present invention, since detection of the intensity of the reflected light requires no drive mechanism, analysis of the sample can be effected at a high speed.